What drives a man to spend 80 hours strapped into a coffin-sized plane cockpit and fly around the world at up to 285 miles an hour?

Multimillionaire adventurer Steve Fossett says his bid to become the first person to circumnavigate the globe in a solo jet plane is not about getting his name in the record books. In any case, his name is already there, in front of a clutch of records for balloon flights. "I do not seek celebrity or adulation - I merely seek respect," he says.

Considering that, perhaps the choice of Sir Richard Branson as reserve pilot for the mission was less than low-profile.

Both men, however, have a lot in common, sharing what Fossett calls "a flair for doing interesting things".

The latest of those is the Virgin Atlantic GlobalFlyer. Currently being tested in California's Mojave desert, it is a unique aircraft for a unique challenge.

Taking more than four years in development, the GlobalFlyer has posed an enormous challenge for its creator, Burt Rutan, of the US aeronautical firm Scaled Composites.

He faced the task of trying to design an aeroplane that was both light enough to get airborne while carrying the 18,000lb fuel payload needed for a 21,000 mile flight, and strong enough to withstand the buffeting of the jet stream.

The result is a remarkable-looking craft, with three fuselages positioned along a giant 114ft wingspan. Its skin is a carbon fibre composite weighing a lot less than the metals used to make normal aircraft, and its livery is painted with a specially designed lightweight paint.

One must-have feature for any long-distance craft - de-icing equipment - has also fallen prey to weight worries, meaning that takeoff (and more problematically, landing) must take place on a cloudless day, when there is no risk of ice forming on the body of the plane.

The timing of the attempt, which is currently scheduled for January next year, is designed so that the craft can piggyback on jet streams, the rivers of fast-flowing air that move high up in the earth's atmosphere, helping to carry it as far and as fast as possible.

GlobalFlyer will take off from the central US, chasing the jet stream across the Atlantic to the UK before heading south to the Mediterranean, through the Gulf, across the Pacific and back towards its start point.

Branson will be trailing in another plane to provide support and act as a reserve pilot. He and Fossett have been here before: during the 90s, they competed against each other to become the first person to fly around the world in a balloon, eventually teaming up to attack the record and becoming friends along the way. Fossett finally succeeded, after six attempts, in 2002.

Both men bubble over with enthusiasm about their latest mission. When Guardian Unlimited spoke to Branson, who had recently returned from a trip to Marrakesh, where he has just bought a kasbah ("We're going to turn it into a wonderful hotel"), he talked about the daunting feats of endurance as though he were popping to the corner shop for a pint of milk, peppering his conversation with the word "incredible".

The flight will raise money for blindness prevention charity Orbis - which has longstanding links with Branson's Virgin Atlantic airline - to fund a paediatric flying eye hospital training programme in India. "We've been involved with Orbis since it began, and one of its principal supporters. It's a flying eye hospital which goes around the world educating doctors into ways of getting people's eyesight back, and it has literally given millions of people their eyesight back," Branson enthuses.

Some might argue that the £1m or so spent by Virgin on GlobalFlyer might be better given directly to Orbis, but Branson is adamant about the mission's value.

"I think, relatively speaking, if you build a new commercial airliner you're going to be spending something like a billion pounds to achieve that," he says. "This project will have cost us something like a million pounds all in. What Burt Rutan is trying to achieve is something incredibly good value for what will be an incredible achievement."

The levels of endurance and single-mindedness needed to spend more than three days in a plane do not seem to trouble the adventurers either.

Fossett, who turned 60 in April, says that - fortunately - the kind of stunts he does "are not terribly age-limiting". His idea of winding down after a gruelling record-breaking attempt would make most people blanch: he returned from sailing around the world with just 19 days' preparation time to run a marathon.

The main problem he will face is staying alert for the duration of the flight. Luckily, he has plenty of experience under his belt, as Branson explains.

"Both of us have been in situations where we've had to be awake for over 80 hours, like our trans-Pacific balloon flight when things went horribly wrong," he says.

"It isn't easy - you can start hallucinating after 30 hours and it can be really rather unpleasant - but Steve and myself as backup will make sure that we're 100% fit and healthy before we do the trip so that we will be able to cope.

"There is a lot to do - you're flying this craft and it's proper flying. You're flying in the jet stream, which is very turbulent, so you're going to be very much awake."

So staying awake and occupied won't be a problem - but what about the delicate issue of relieving yourself? Branson reveals that, during a previous balloon flight, he and Fossett had hit on an ingenious solution to the tricky problem of disposing of urine-filled bags.

"We would choose whichever countries had been unpleasant to us when we asked permission to fly over them," he recalls. "I think Iraq got one load and somewhere else got another - maybe China got the other load."

Urine payloads aside, there is a very real risk that something could go terribly wrong.

GlobalFlyer will, during the course of its journey, move from being fuel laden to incredibly light, which - when added to the turbulence of the jet stream - will make for an unpredictable flight, particularly during takeoff and landing.

As Popular Science magazine unsentimentally put it: "If things do not go well, Fossett's flight will end in a horrific fireball." For once, Branson doesn't sound too displeased about putting someone else in the hotseat.

"Always with adventures, you have mixed feelings about them ... Steve doesn't have children and I do, and I suspect I've used up most of my nine lives over the past few years," he says. "So I suspect it will most likely be a good thing if Steve ends up doing it, and not myself."

The plane will have made around 35 test flights before taking to the skies for the record attempt, but Fossett says he will still be "the ultimate test pilot", because the plane will not have been flown with its full complement of fuel.

However, neither man is a stranger to being rescued from remote locations. "I think Steve's been rescued four or five times out of the sea in his life, and I've been rescued five times by helicopter so we're got experience, I'm afraid, of sea rescues," Branson says.

One thing is certain: whether the mission ends in failure or success, the drama will make a great story, mark a milestone in aviation history and, not least for Branson, provide unrivalled publicity for his Virgin Atlantic brand.

As he puts it, "We've got wonderful stories to tell our grandchildren: incredible experiences which - if I survive them - I can share."